Abstract
Monsoon depressions are synoptic‐scale features that are responsible for a significant fraction of the rain over northern India during the summer monsoon season, and therefore it is important to quantify their structure and behaviour in numerical weather prediction (NWP) models. It is known that increasing model resolution is strongly correlated with improved forecasts in the short term and global circulation in the longer term, as well as better representation of tropical cyclones; here, we explore the sensitivity of depressions to changes in resolution using the Met Office Unified Model. Seven NWP case‐studies of depressions from 2013 to 2015 were run at eight resolutions corresponding to equatorial grid spacing of between 16 and 208 km, and compared with data for the same events from Tropical Rainfall Measuring Mission (TRMM) and ERA‐Interim reanalysis. We found that, at the low‐resolution end of the spectrum, increases in resolution led to improvements in the composite structure, but with diminishing returns; that is to say, the improvements in forecast track and structure become smaller. The model also persistently overestimated the depression intensity, in particular the wind speed and the warm core aloft–with the source appearing to originate in the mid‐troposphere. The sensitivity of the diurnal cycle to resolution was also explored: the stratiform component was found to be very well represented by the model, whereas the convective component was described quite poorly. Improvement in most components of structure with increasing model resolution were marginal beyond N320 (63 km) and N512 (39 km) for dynamic and thermodynamic fields respectively.
Highlights
Indian monsoon depressions (MDs) are synoptic-scale systems that typically spin up over the Bay of Bengal before propagating northwestward over the Indian subcontinent and terminating over northwest India or Pakistan (Godbole, 1977; Hurley and Boos, 2015; Hunt et al, 2016a)
That we have explored in some detail the sensitivity of a depression-trough to changes in horizontal model resolution, we shall look at the performance of the models from an MDcentred point of view
The first field under consideration (Figure 10(a)) is the lowest central surface pressure associated with the MD during its lifetime: parallel to similar studies for tropical cyclones (e.g. Strachan et al, 2013) we see that increasing the resolution strengthens the minimum surface pressure associated with the system irrespective of appropriate downsampling, though even at some of the lowest model resolutions (N216: 989.7 hPa), it is still lower than the values suggested by ERA-I reanalysis, and deepens substantially beyond this (N1280: 984.8 hPa)
Summary
Indian monsoon depressions (MDs) are synoptic-scale systems that typically spin up over the Bay of Bengal before propagating northwestward over the Indian subcontinent and terminating over northwest India or Pakistan (Godbole, 1977; Hurley and Boos, 2015; Hunt et al, 2016a). Despite the otherwise highly favourable conditions, strong vertical wind shear during the Indian monsoon prevents depressions from further deepening into tropical cyclones; they still bring significant and occasionally devastating precipitation to agrarian north India, and are responsible for modulating the majority of the seasonal (June–September) rainfall there (Mooley, 1973). They have an average duration of around 5 days, and generally occur between two and four times per season. Durations than the official MDs declared by the IMD due to our weaker wind criterion (
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